1. Field of the Invention
The present invention relates to a multilayer molded article and a method for producing the same. In particular, the present invention relates to a thermoplastic resin molded article having a skin material layer, which article has good appearance and is used as an automobile interior panel or electric appliances, and a method for producing such molded article.
2. Description of the Related Art
There are widely known high-grade molded articles which have improved beauty and softness by laminating a skin material thereon. As the skin material, a film or sheet of soft polyvinyl chloride or a thermoplastic elastomer which is backed with a foam layer of polyolefin or polyurethane resin to give a cushioning effect is used.
As a method for producing a multilayer molded article having a skin material, there is a method comprising forming a molded article of a thermoplastic resin by a conventional molding method such as injection molding and then forming the skin material on the molded article by, for example, vacuum molding, or simultaneously forming the molded article and the skin material with laminating the skin material to the article with an adhesive.
Alternatively, the multilayer molded article can be produced by placing the skin material in a mold, closing the mold, then injecting a molten thermoplastic resin in a mold cavity through a gate of the mold to laminate the resin with the skin material, cooling the mold and opening the mold to remove the molded article.
However, each of the above methods has its own drawbacks.
Since the polyvinyl chloride skin material has a comparatively large specific gravity and also poor weather resistance, when it is used as a skin material of an article to be used in direct rays of the sun, its surface appearance is greatly deteriorated. Since polyvinyl chloride is softened by the addition of a large amount of a plasticizer, when it is used as the automobile interior panel, it will cloud a front glass due to the influence of the plasticizer. Since polyvinyl chloride will generate hydrogen chloride gas when it is burnt because of the presence of chlorine atoms in its chemical structure, it cannot be destroyed by burning in view of environmental pollution, or it will corrode various processing apparatuses.
Though the thermoplastic elastomer does not have the above drawbacks of polyvinyl chloride when used as the skin material, it is easily flawed so that the appearance is gradually deteriorated as the article is used.
With respect to the production method, the method utilizing the adhesive is complicated so that the molded article becomes expensive. In addition, a solvent, a dispersant and the like used in the adhesive tend to pollute the environment. In the injection molding method, since the resin melt is injected in the closed mold so that the resin melt at high temperature and high pressure contacts to the skin material and presses the skin material to the mold wall strongly, the surface of the skin material is discolored and designs of the skin material are deformed, so that the appearance of the molded article is deteriorated. When the skin material backed with the foam layer is used, the foam layer is crushed or damaged by the same reason, so that the cushioning effect and appearance of the molded article are decreased. When the thermoplastic elastomer is used as the skin material, such defective moldings are often found.
To prevent such defective appearance of the molded article, it is proposed to wet the surface of the skin material (cf. Japanese Patent Kokai Publication No. 61214/1989), or to suppress increase of a temperature or a degree of deformation of the skin sheet by increasing a tensile strength of a backing material (cf. Japanese Patent Kokai Publication Nos. 159342/1984 and 22545/1989).
However, further processing of the already prepared skin material such as wetting of the skin material adds one processing step to the production method and increases a production cost. To increase the tensile strength of the skin material, it is contemplated to increase a thickness of the backing material, to bond an additional discrete sheet to the skin material, or decrease an expansion ratio of the foam layer. However, these measures will increase material costs. Further, because the skin material is placed between upper and lower molds, if the same molds used before the increase of the skin material thickness are used in molding, a distance between the upper and lower molds is increased by the use of the thicker skin material, so that a larger amount of the resin melt should be supplied in the mold cavity so as to fill every corner of the mold cavity. Therefore, to produce a satisfactory multilayer molded article, an amount of the thermoplastic resin or the pressure for molding should be increased. As a result, the material cost or a weight of the produced article increases, or the appearance of the article may be deteriorated. If the mold design is modified or changed to overcome such a problem, a cost for such modification is required.
In the mold cavity in which the material is influenced by heat, pressure or stretching, the skin material which is backed with the foam layer and has an outermost layer of the thermoplastic elastomer sheet suffers from excessive deformation of the foam layer, so that the foam layer is crushed, whereby the appearance of the molded article is deteriorated or the skin material is broken.